The invention relates to a propulsion device, in particular an underwater propulsion device, for divers and swimmers, said device being provided with one or more drives comprising electric motors, and having means for fixing the drives to the legs or arms of the diver or swimmer.
For faster and fatigue-free movement of a diver under water, diver propulsion vehicles (DPV) are known from the state of the art. They usually have a torpedo shape and are held by the diver with his hands so that the DPV tows the diver through the water. These devices have proved their worth in principle, but the flexibility of the divers is very limited because they can no longer use their arms and hands freely, for example to communicate with other divers, to let air into or out of the jacket (buoyancy compensator), hold a diving lamp, operate the diving computer, take photographs or grab or take hold of objects under water. For this reason, underwater propulsion devices have already been proposed which are attached to the legs, especially to the upper or lower leg of the diver, for example in publications U.S. Pat. Nos. 6,823,813 B2 and 8,567,336 B1.
The drives of dive scooters or underwater propulsion systems call for a comparatively high torque at relatively low speed rates, which is due to the hydrodynamic requirements to be met to enable the propeller to operate efficiently. Therefore, permanently excited synchronous motors are typically used in conventional DPVs, especially internal rotor motors with speed reduction gear. Some internal rotor motors without gears are also known but require more installation space to produce the same torque.
In these drives, the externally arranged stator is often thermally and conductively connected to the inner wall of the housing via the outer shell surface. Since water continuously washes around the housing and due to the fact that the housing of the drive is relatively large, an adequate cooling effect is achieved. The internally mounted rotor of the electric motor is cooled solely by thermal convection and radiation.
The use of drives that are attached to the upper or lower leg of the diver now causes the problem that the surface available for cooling by means of the surrounding water is significantly smaller than the surface available with conventional DPVs. Accordingly, a more effective cooling system is needed.
One possibility to achieve an efficient cooling effect, but for ship propulsion systems, which is described in the German utility model DE 20 2011 109 894 U1, is a flooded electric motor that is continuously flown through by the surrounding liquid. In this way, a very good cooling effect is brought about that is sufficient even for electric motors located inside a small housing. In this context, however, it is problematic that the rotor and stator are continuously exposed to highly corrosive salt water and impurities in the water, especially when diving in the sea. To counteract this, these components must be provided with a polyurethane resin coating or a protective sheathing or cladding consisting of titanium, stainless steel or fiberglass or carbon reinforced plastic material, for example. The use of conventional antifriction bearings is not possible so that water-lubricated plain bearings must be employed instead. However, these cause higher friction so that greater losses are to be put up with.
Another possibility is the use of an oil-filled electric motor, as described in DE 39 16 253 A1 for submersible pumps which are designed for high-temperature operation with a view to pumping oil or water out of boreholes. In contrast to the variant described hereinbefore, such an electric motor is provided with a watertight housing that is completely filled with oil. However, since the oil heats up during operation a change in volume occurs, so that special pressure equalizing reservoirs have to be provided. Moreover, as a result of the continuous rotation of the components within a viscous fluid, relatively high friction is encountered. Although friction can to some extent be compensated for by providing particularly smooth surfaces and high rotational symmetry of the motor components, this in turn increases the effort involved in manufacturing the electric motor components. What is more, due to the complete oil filling the housing of the drive is unable to produce any buoyancy effect.
Proceeding from what is known from prior art as described hereinbefore it is therefore the objective of the invention to propose and provide a propulsion device that can be attached to the legs or arms of a diver or swimmer and enables sufficient cooling to be produced at high drive power and does not have the previously described disadvantages associated with the state of the art.